The invention relates to static power frequency changers in general, and more particularly to Unrestricted Frequency Changers (UFC) and their applications, for instance to adjustable speed AC motor drives.
The Unrestricted Frequency Changer (UFC) and its adjunct static switch control for the generation of an AC wave of controlled voltage and frequency have been described in U.S. Pat. Nos. 3,470,447 and 3,493,838 of L. Gyugyi et al. These patents show how the switches in each of the static converters associated with an output phase of the load can be selectively and cyclically controlled for conduction during a predetermined time interval so as to derive and output power defined by a controlled increment of the input voltage, itself delineated between two time intervals are used for shorting the output, which process results in an AC output voltage having a frequency depending upon the repetition rate of the conduction time intervals and a magnitude measured by the time period of effective conduction of each static switch. Such an unrestricted frequency changer is advantageously applied in variable speed AC drives as explained on pages 5-14, and 363-383 of "Static Power Frequency Changers" by L. Gyugyi and B. R. Pelly, published by John Wiley & Sons 1976. In this regard, for instance, Gyugyi and Pelly have observed that the UFC has an inherent bilateral characteristic between the powe source at its input and the power supply at its output, which allows a four-quadrant operation of the motor drive without costly additional circuitry.
The unrestricted frequency changer technique has become particularly attractive with the advent of modern bilateral switches, for instance, power transistors, and GTO devices.
When used for controlling the speed of an AC motor, the Unrestricted Frequency Changer requires a voltage source type termination at the input terminals. This is due to the fact that the AC motor represents an inductive load at the output. Since the UFC connects the multi-phase input source sequentially to such inductive load (thus, load current), the input source must provide low impedance path for the step-like current wave drawn by the UFC. The voltage source type termination in practice is normally provided by a low pass LC input filter in such a way that the input terminals of the UFC are shunted by capacitors. This type of filter, viewed from the input terminals of the UFC, constitutes a parallel connected LC circuit, which provides a low impedance for the high frequency (harmonic) components present in the step-like UFC input current wave at frequencies higher than the resonant frequency of the circuit. Thus, the basic criterion for the design of the filter is to ensure that its resonant frequency (or "cut-off" frequency) is lower than the lowest input harmonic frequency (corresponding to the minimum output frequency) so as to avoid resonant amplification of the input current harmonics. This criterion often necessitates a relatively large and expensive input filter.
The Unrestricted Frequency Changer (UFC) is an AC-to-AC converter. In addition to the fundamental current, extrabasal currents are generated by the UFC flow through the input power line. Therefore, a properly designed input filter is essential to the pulse-width modulated UFC-induction motor drive system. A method of filter design is known for a double pulse-width modulated 6-pulse UFC. The most difficult problem in the filter design, though, is that the low frequency extrabasal components, where .omega..sub.I is the angular input frequency and .omega..sub.O is the angular output frequency, namely the (5.omega..sub.I +6.omega..sub.O) and (7.omega..sub.I +6.omega..sub.O) components, in the case of a 6-pulse UFC, are hard to eliminate. By increasing the modulation frequency, the magnitudes of these two low frequency components can be lowered to some extent. However, they still remain no matter how high the modulation frequency can be increased. Another method of reducing the total rms extrabasal current is to modulate each of the three UFC converters individually so that between the output phases the control pulses are "interlaced", so as to minimize the overlap among neighboring pulses on the input side. This is the approach disclosed in concurrently filed copending patent application Ser. No. 06/596,329.
A preferable method would be to eliminate both the (5.omega..sub.I +6.omega..sub.O) and the (7.omega..sub.I +6.omega..sub.O) components, in a 6-pulse UFC situation. Although a specific example that it is possible does exist, it is not conceivable, at this time, that those two extrabasal components can be always eliminated for the whole range of the output voltage.
Assuming for the sake of discussion that elimination of only one of the two components, instead of two, in the UFC is possible, it will be either the (5.omega..sub.I +6.omega..sub.O) component, or the (7.omega..sub.I +6.omega..sub.O) component. Furthermore, if it is assumed that the relative amplitudes of those two components is being controlled, it becomes conceivable, then, to increase the cut-off frequency of the low-pass filter substantially thereby to reduce the filter size and the VAR rating considerably. In other words, it is possible to avoid, or at least mitigate, the effect of the filter resonance by controlling the extrabasal components which are near to the cut-off frequency of the filter. Nevertheless, when reducing one component, the amplitude of the other component is generally increased. In conclusion, this leaves still open the requirement of a total harmonic minimization method for a given filter characteristic.